1. Field of the Invention
The present invention relates to a zoom lens and more particularly to an improved zoom lens especially suitable for a video camera.
2. Related Background Art
As compared with zoom lenses for a 35 mm camera, zoom lenses for a video camera generally have a larger aperture ratio and a larger zoom ratio. Such zoom lenses for a video camera well-known in the art are of the type comprising four lens groups. Of the four groups, the first one, as viewed from the object side, is a lens group of positive refractive power which remains stationary during zooming but moves for focusing. The second lens group has a negative refractive power and is movable for the change of magnification. The third lens group has a positive or negative refractive power and is moved for keeping the position of the image plane constant. The fourth lens group is a lens group fixed for image formation and has a positive refractive power.
For the zoom lens for a video camera, on the other hand, it has become desirable to reduce the size and increase the zoom ratio further. However, the above-mentioned type of known zoom lenses have some difficulties in answering these demands. In the known zoom lens for a video camera, only the second and third lens groups are moved for zooming. This type of zoom lens has a large variation of aberrations by zooming. It is very difficult to correct the variation of aberrations in a well-balanced manner and to keep good image-forming performance throughout the changing range of magnification. These difficult problems have prevented the realization of zoom lenses having a smaller size and a larger zoom ratio.